1. Field of the Invention
The invention relates to a high-speed rotary spray head specifically usable for electrostatic spraying of paint or varnish; it is more particularly concerned with a new system of suspending the rotary shaft carrying the spray disk or cup.
2. Description of the Prior Art
There are known electrostatic paint application devices comprising a spray member in the shape of a disk or cup rotating at high speed and held at a high electrical voltage relative to ground. The liquid to be sprayed is deposited on the disk or in the cup and divided into fine particles as a result of the electrical and centrifugal forces applied to it. The charged particles are attracted by the structure to be covered, which is electrically grounded. The directional properties of the beam of particles may be enhanced by a ring of air jets to the rear of the spray member. To obtain a good surface finish on the structure it is essential to reduce the particle size as much as possible. Other things being equal, this means that it is necessary to increase the speed of rotation of the spray member. According to the type of spray head, this speed varies widely, between values in the order of 1000 and 100,000 rpm, for example.
In one well-known technique the shaft carrying the spray disk or cup is supported by ball or roller bearings. These procure both radial suspension of the shaft and its axial stabilization within the body of the spray head. Lubricated for life bearings limit the service life of the device to a few thousand hours.
Attempts have been made to lubricate permanently these bearings by means of a mist of oil mixed with air, in order to increase the service life. This type of arrangement raises the problem of avoiding any leakage of oil which could mix with the paint and spoil the finish of the painted surface, however.
Also, for a given system of lubrication the service life of the bearings is inversely proportional to the tangential speed of the mobile part of the bearing relative to its fixed part. It has therefore been necessary to reduce the diameter of the bearings and consequently that of the shaft driving the spray member. This approach encounters technological limits when high rotation speeds are to be obtained. Specifically, it becomes practically impossible to feed the product to be sprayed through an axial feed bore in a hollow shaft. For this reason lateral feed systems for the products to be sprayed have been proposed.
In the remainder of this description the term "bearing" will be used for any arrangement procuring (or contributing to the procurement of) suspension and radial guidance of the shaft and the term "thrust bearing" wil be used for any arrangement procuring axial positioning relative to the body of the spray head. Any bearing or thrust bearing is characterized by its stiffness which is the quotient of the force exerted on the shaft divided by the resulting displacement. The stiffness of ball and roller bearings is high and they can withstand considerable loads without significant displacement resulting. As the mechanical axis of the shaft is not always coincident with its principal axis of inertia, however, the resulting imbalances produce very high forces at the bearing races and the balls or rollers. Lubrication is seriously affected to the degree that in some cases these imbalances lead to rapid destruction of the bearings. Also, the bearing nearest the spray member is generally inadequately protected from atomized paint emitted in its proximity.
Attempts have recently been made in this art to eliminate roller bearings at least to some degree. Various systems have been put forward to fulfill the two essential functions of these bearings: suspension and radial guidance of the shaft, on the one hand, and axial positioning of the shaft within the body, on the other hand.
There is one known arrangement in which these two functions are implemented by fluidic bearings and by at least one fluidic thrust bearing. Gas, generally air, is injected under pressure between radial and axial bearing surfaces defined on the shaft and within the body of the spray head. This gas flow results in complete suspension of the shaft in the body, with which there is no direct mechanical rubbing contact. This reduces wear and makes it possible to rotate the shaft at high speed without it being necessary to reduce the diameter of the shaft. It is therefore possible to feed the product to be sprayed axially. For the system to be truly effective, however, the clearance between the fixed and rotating parts must be extremely small (in the order of a few microns). These parts therefore have to be machined particularly carefully. Also, the limited radial clearance available permits only small displacements of the shaft relative to its theoretical axis of rotation. The risks of contact between the shaft and the various bores in which it is accommodated are thus relatively high, especially if during use imbalances or instability and resonance phenomena occur. Finally, the air and therefore energy consumption are high, especially in connection with the fluidic bearings.
With specific relevance to the problem of radial suspension, another system as described in U.S. Pat. No. 4,378,091 proposes a combination of main fluidic bearings self-fed by natural aspiration of air and an auxiliary magnetic bearing. The function of a bearing of this kind is primarily to assist the fluidic bearings at low rotation speeds and to prevent contact between the parts on starting up and shutting down the device. In this magnetic bearing the magnetization is radial and the bearing is therefore implemented by two cooperating and interpenetrating magnets superposed radially. The low stiffness of the magnetic suspension cannot be exploited in a system of this kind to counter the instability phenomena mentioned above since there is a critical clearance where the fluidic bearings are concerned.
The present invention proposes an improvement on this arrangement associated with a new structure magnetic bearing.